pool_setlowat man page on OpenBSD

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POOL(9)			     OpenBSD Kernel Manual		       POOL(9)

NAME
     pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_setipl,
     pool_sethiwat, pool_setlowat, pool_sethardlimit - resource-pool manager

SYNOPSIS
     #include <sys/types.h>
     #include <sys/pool.h>

     void
     pool_init(struct pool *pool, size_t size, u_int align, u_int
     align_offset, int flags, const char *wmesg, struct pool_allocator
     *palloc);

     void
     pool_destroy(struct pool *pp);

     void *
     pool_get(struct pool *pp, int flags);

     void
     pool_put(struct pool *pp, void *item);

     int
     pool_prime(struct pool *pp, int nitems);

     void
     pool_setipl(struct pool *pp, int ipl);

     void
     pool_sethiwat(struct pool *pp, int n);

     void
     pool_setlowat(struct pool *pp, int n);

     int
     pool_sethardlimit(struct pool *pp, unsigned n, const char *warnmess, int
     ratecap);

DESCRIPTION
     These utility routines provide management of pools of fixed-sized areas
     of memory.	 Resource pools set aside an amount of memory for exclusive
     use by the resource pool owner.  This can be used by applications to
     guarantee the availability of a minimum amount of memory needed to
     continue operation independent of the memory resources currently
     available from the system-wide memory allocator (malloc(9)).  The pool
     manager obtains memory by using the special-purpose memory allocator
     palloc passed to pool_init(), for extra pool items in case the number of
     allocations exceeds the nominal number of pool items managed by a pool
     resource.	This temporary memory will be automatically returned to the
     system at a later time.

   CREATING A POOL
     The function pool_init() initializes a resource pool.  The arguments are:

	   pool		 Specifies the pool storage to be initialized.

	   size		 Specifies the size of the memory items managed by the
			 pool.

	   align	 Specifies the memory address alignment of the items
			 returned by pool_get().  This argument must be a
			 power of two.	If zero, the alignment defaults to an
			 architecture-specific natural alignment.

	   align_offset	 The offset within an item to which the align
			 parameter applies.

	   flags	 Specifies various flags set on the pool at creation
			 time.

	   wmesg	 The message passed on to tsleep(9) if pool_get() must
			 wait for items to be returned to the pool.

	   palloc	 The back-end allocator used to manage the memory for
			 the pool.  palloc may be NULL, in which case the pool
			 manager uses an interrupt safe allocator.  It is
			 recommended that this be set to pool_allocator_nointr
			 if the pool will never be accessed in an interrupt
			 context.

   DESTROYING A POOL
     The pool_destroy() function destroys a resource pool.  It takes a single
     argument pp identifying the pool resource instance.

   ALLOCATING ITEMS FROM A POOL
     pool_get() allocates an item from the pool and returns a pointer to it.

	   pp	  The handle identifying the pool resource instance.

	   flags  One or more of PR_URGENT, PR_WAITOK, PR_NOWAIT or
		  PR_LIMITFAIL, that define behaviour in case the pooled
		  resources are depleted.  If no resources are available and
		  PR_WAITOK is given, this function will wait until items are
		  returned to the pool.	 Otherwise pool_get() returns NULL.
		  PR_NOWAIT should be provided in cases where PR_WAITOK is
		  not.	If PR_URGENT is specified and no items are available
		  and palloc cannot allocate a new page, the system will panic
		  (XXX).  If both PR_LIMITFAIL and PR_WAITOK are specified,
		  and the pool has reached its hard limit, pool_get() will
		  return NULL without waiting, allowing the caller to do its
		  own garbage collection; however, it will still wait if the
		  pool is not yet at its hard limit.  If PR_ZERO is passed and
		  an item has been successfully allocated, it is zeroed before
		  being returned to the caller.

   RETURNING ITEMS TO A POOL
     pool_put() returns the pool item pointed at by item to the resource pool
     identified by the pool handle pp.	If the number of available items in
     the pool exceeds the maximum pool size set by pool_sethiwat() and there
     are no outstanding requests for pool items, the excess items will be
     returned to the system by calling prelease().

	   pp	 The handle identifying the pool resource instance.

	   item	 A pointer to a pool item previously obtained by pool_get().

   PRIMING A POOL
     pool_prime() adds items to the pool.  Storage space for the items is
     allocated by using the page allocation routine specified to pool_init().

     pool_prime()

	   pp	   The handle identifying the pool resource instance.

	   nitems  The number of items to add to the pool.

     This function may return ENOMEM in case the requested number of items
     could not be allocated.  Otherwise, the return value is 0.

   SETTING POOL RESOURCE WATERMARKS
     A pool will attempt to increase its resource usage to keep up with the
     demand for its items.  Conversely, it will return unused memory to the
     system should the number of accumulated unused items in the pool exceed a
     programmable limit.  The limits for the minimum and maximum number of
     items which a pool should keep at hand are known as the high and low
     watermarks.  The functions pool_sethiwat() and pool_setlowat() set a
     pool's high and low watermarks, respectively.

     pool_sethiwat()

	   pp	  The handle identifying the pool resource instance.

	   n	  The maximum number of items to keep in the pool.  As items
		  are returned and the total number of pages in the pool is
		  larger than the maximum set by this function, any completely
		  unused pages are released immediately (by calling
		  prelease()).	If this function is not used to specify a
		  maximum number of items, the pages will remain associated
		  with the pool until the system runs low on memory, at which
		  point the VM system will try to reclaim unused pages.

     pool_setlowat()

	   pp	  The handle identifying the pool resource instance.

	   n	  The minimum number of items to keep in the pool.  The number
		  of pages in the pool will not decrease below the required
		  value to accommodate the minimum number of items specified
		  by this function.  Unlike pool_prime(), this function does
		  not allocate the necessary memory up-front.

   SETTING THE PROTECTION LEVEL
     The pool_setipl() function is used to specify the interrupt protection
     level at which the pool can be safely used.

     pool_setipl()

	   pp	  The handle identifying the pool resource instance.

	   ipl	  The interrupt protection level used to protect the pool's
		  internals.  See spl(9) for a list of the IPLs.

   SETTING HARD LIMITS
     The function pool_sethardlimit() sets a hard limit on the pool to n
     items.  If the hard limit is reached warnmess will be printed to the
     console, but no more than every ratecap seconds.  Upon successful
     completion, a value of 0 is returned.  The value EINVAL is returned when
     the current size of the pool already exceeds the requested hard limit.

   POTENTIAL PITFALLS
     Note that undefined behaviour results when mixing the storage providing
     methods supported by the pool resource routines.

     The pool resource code uses a per-pool lock to protect its internal
     state.  If any pool functions are called in an interrupt context, the
     caller must block all interrupts that might cause the code to be
     reentered.

   DEBUGGING
     To debug a misbehaving pool, a kernel can be compiled with the
     MALLOC_DEBUG option and memory debugging on pools can be enabled with the
     PR_DEBUG flag passed in the flags argument in the call to pool_init().
     See malloc(9) for more information about MALLOC_DEBUG.

CODE REFERENCES
     The pool manager is implemented in the file sys/kern/subr_pool.c.

SEE ALSO
     free(9), malloc(9), spl(9), uvm(9)

HISTORY
     The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD
     by Artur Grabowski <art@openbsd.org>.

OpenBSD 4.9			 March 8, 2010			   OpenBSD 4.9
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